I discovered this amazing service the other day, I was aware of the OpenStreetMap project, user contributed global street maps, but I was unaware of the additional service garmin.openstreetmap.nl which allowed you to download maps to be used on Garmin devices, including MapSource. Whilst Garmin produces some very comprehensive country maps, they miss some of the rarer countries, which OpenStreetMap have covered.
For information only, How to Unlock Garmin maps.
First get your hands on the desired Garmin City Navigator Downloadable Update for your country/city (MapSource A).
In the compressed file should be the folders IMG, support and Windows plus a readme.txt.
You will need a version of Garmin MapSource 6.13.1 or older which does NOT support Unlocking via the internet (MapSource B).
This can be found in older version of the downloaded map update files (perhaps a 2007/2008 version).
Run the CN.msi installation file to install the old version of MapSource (B).
Run MapSource (B) and the UnlockWizard.
Now get your hands on Garmin Unlock Generator V1.5,
Enter your 10 digit Unit ID, and select map product <custom mapset>, enter the FID (MapID) of the newest Downloadable Update (the one you want to Unlock).
Click generate, copy the 25 digit map unlock code and paste this into the UnlockWizard.
At this stage you will have the older version of MapSource (B) with some old map.
Run the CNMENT2010Update_ENU.msi file in IMG folder (MapSource A).
Run MapSource (A). That’s it. The map should now be Unlocked.